Chemical vapor deposition (hereinafter referred to as "CVD") is known as a method of coating metals or ceramics upon the material to be processed. In CVD, a gaseous composition (e.g. reaction gas+carrier gas or the like) is supplied to a heated material to be processed and causes the reaction gas to contact surfaces thereof and precipitate thereon contents in the gaseous composition by chemical reactions. CVD has broadly been employed, since it allows various kinds of coatings and materials to be processed and has a lot of excellent merits such as good adhesion property.
However, CVD has had problems that the depositing speed is low, and so it takes much time for treatment. Therefore, if it were applied to a continuous line such as a continuous treatment line of a steel strip, any treating furnace would be considerably lengthened. Further in the CVD method, the deposited film easily becomes irregular in thickness, and this is especially noticeable in continuous treatment lines. Thus, it has been actually difficult to apply CVD to continuous treatment lines.
The inventors have made studies of the depositing speed and the irregular thickness of the deposited film in the case of conventional CVD treatments. As a result, they have found that those problems were closely related to the fluidity of the gas in the vicinity of the reacting surface of the material being treated. The prior art taught that, if the gas composition in CVD were made to flow rapidly, depositing irregularities would be caused, air bubbles would be formed in or mixed into the deposited film, further purity of the deposited film would be lowered, and accordingly, the gas fluidity had to be controlled to a necessary minimum. However, it was found through the inventors' studies that, as a result of suppressing the flowing of the gas to avoid the aforementioned problems, different problems arose: namely, diffusion of the reaction gas to the surface of the material being processed could not be made smooth, and a separation of reacted by-product (gas produced by reaction) from the surface larger thereof could not be made smooth, so that it took much time for the treatment; moreover the concentration of the reaction gas within the treating chamber was effected with non-uniform distributions, and consequently the deposited film was made irregular in thickness.
In view of these problems, it is an object of the invention to provide a method for performing the CVD treatment on the steel material in the continuous line without making the line lengthened or causing the deposited film to be irregular in thickness.
Having made further studies on the above mentioned problems, the inventors found that, if a reaction gas or a gas containing a reaction gas were blown to the steel material through blowing nozzles in the CVD treating chamber, the treatment could be accomplished at the high depositing speed, while suppressing the irregular thickness of the deposited film.